This is an outdated form of a specific hydrogen bonding term. (Hydrogen bonding can be properly modeled with a combination of electrostatics and van der Waals terms.)

The abhb term is the sum of N_abhb terms, each of the form:

H[ rinclude - rij ] { A / rij12 - B / rij10 }
   

where r_ij is the distance between the atoms atom_i and atom_j, A and B are constants and H[r] is the Heaviside step function in which H[r<0] = 1, H[r=0] = 1/2 and H[r>0] = 0. The function is abruptly terminated at r_include. The abhb record in the descriptor has the form

abhb	Npair	rinclude	rcutoff
	atomi	A	B	Nij
		atomij	...
	...	...	...	...

The abhb record starts with the keyword abhb followed by the number of pair lists N_pair, the distance at which the function will be terminated r_include and the cutoff distance r_cutoff, followed by N_pair lists. Each list starts with an atom number atom_i, followed by the parameters A and B and the number of atoms that interact with atom_i with these parameters N_ij and finally the N_ij atom numbers atom_ij, ... The sum of all the values of N_ij equals N_abhb. r_cutoff has to be larger than r_include. The calculation of the abhb term is done for a subset of the full list, from a candidate list. The candidate list contains atom pairs that are within a distance r_cutoff apart. This list is updated regularly. In between updates, some of the atom pairs in the candidate list move apart, while pairs of atoms that are not in the candidate list move closer together. The distance r_cutoff and the list update interval are chosen to reduce computation by reducing the difference between r_include and r_cutoff and by increasing the interval between list updates. These are mutually incompatible. Example:

abhb   2   9.0   12.0

3   8561000   2568000   3
    45  15  57
4   62290000   11960000   2
    15  35

There are two inclusion lists, the first has three pairs involving atom 3, the second has two pairs involving atom 4. The first set of hydrogen bond has the energy minimum at 2Å while the second minimum is at 2.5Å. r_include is 9Å which is perhaps inadequate given the positions of the minima.